The invention relates to an apparatus for plasticising and injecting plastic material comprising a rotatable screw arranged in a cylinder, wherein the screw is displaced axially by the rotation of spindles which are rotatable by a servomotor moved axially with the screw, by means of gears connected by toothed belts.
A known apparatus of that kind (see EP 1 151 843 A1) was apparently logically constructed with the aim that the masses which are moved upon injection of the plastic material are as small as possible and the structure overall is kept as simple as possible. That is achieved in that structure shown in FIGS. 1 to 3, insofar as the drive output pinion of the servomotor is connected by means of a toothed belt directly to a respective gear which sits directly on each of the spindles. In order to be able to keep the motor relatively small it is caused to rotate at high speed and that is stepped down by the use of a small drive output pinion and a large drive gear.
Although the known design functions satisfactorily, the invention provides that an intermediate transmission for reducing the rotary speed is arranged between the servomotor and the gears.
Initially it is not clear why that increased structural complication and expenditure should be more than offset by an overall improved function of the entire apparatus. With the same number of revolutions of the servomotor and the spindles, the arrangement in accordance with the invention firstly reduces the speed of the toothed belt according to the transmission ratio of the intermediate transmission. That provides that the installation is substantially quieter in operation. That advantage alone however would scarcely justify the increased structural complication and expenditure, especially as the intermediate transmission provided contributes more to the moving mass than is saved by the reduction in the size of the gears which are carried on the spindles. Nonetheless the novel construction contributes to making the injection procedure more easily controllable as it has been found that it is not the inertia of the axially moved masses alone but to a substantial extent the mass moments of inertia of the rotating components of the machine that are responsible for that. Ultimately therefore the invention even makes it possible to use a smaller but faster running servomotor with nonetheless substantially improved control of the operating procedure involved.